1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device that has a high brightness.
2. Description of Related Art
Until now, the cathode-ray tube (CRT) has been developed for and is used mainly for the display systems. However, the flat panel display is beginning to make its appearance due to the requirements of small depth dimensions, undesirably low weight and low voltage power supply. At present, the thin film transistor-liquid crystal display (TFT-LCD) with high resolution and small depth dimension has been developed.
During operation of the TFT-LCD, when the pixel is turned ON by the corresponding switching elements, the pixel transmits light generated from a backlight device. The switching elements are generally amorphous silicon thin film transistors (a-Si:H TFTs) which use an amorphous silicon layer. Advantageously, the amorphous silicon TFTs can be formed on low cost glass substrates using low temperature processing.
In general, the TFT-LCD transmits an image using light from the backlight device that is positioned under the TFT-LCD panel. However, the TFT-LCD only employs 3˜8% of the incident light generated from the backlight device, i.e., the inefficient optical modulation.
Referring to FIGS. 1-5B, a TFT-LCD device that is manufactured by a conventional method will now be explained in some detail.
FIG. 1 is a graph illustrating a light transmittance respectively measured after light passes through each layer of a conventional liquid crystal display device. The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive TFT-LCD device has a transmittance of about 7.4% as seen in FIG. 1, which shows a transmittance after light passes through each layer of the device. For this reason, the transmissive TFT-LCD device requires a high, initial brightness, thereby increasing electric power consumption of the backlight device. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. Moreover, there still exists a problem that the battery cannot be used for a long time.
In order to overcome these problems, a reflective TFT-LCD has been developed. Since the reflective TFT-LCD device uses ambient light, it is light and easy to carry. Also, the reflective TFT-LCD device is superior in aperture ratio as compared to a transmissive TFT-LCD device. Namely, since the reflective TFT-LCD substitutes an opaque reflective electrode for a transparent electrode material in the pixel of the conventional transmissive TFT-LCD, it reflects the ambient light.
As described above, since the reflective TFT-LCD device uses ambient light other than an internal light source such as a backlight device, battery life can be increased resulting in longer use times. In other words, the reflective TFT-LCD device is driven using light reflected from the reflective electrode. Thus, only the drive circuitry that drives the liquid crystal uses the battery power in the reflective TFT-LCD device.
Additionally, the reflective TFT-LCD device has a problem that it is affected by its surroundings. For example, the brightness of indoors-ambient light differs largely from that of outdoors-ambient light. Also, even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk). Therefore, the reflective TFT-LCD device cannot be used at night without ambient light.
Accordingly, there is a need for a transflective TFT-LCD device that can be used during daytime hours as well as nighttime because the transflective LCD device can be changed to either a transmissive mode or a reflective mode depending on the desired condition of operation.
FIG. 2 is a schematic cross-sectional view illustrating one pixel of the transflective TFT-LCD device according to the conventional art. As shown, the transflective TFT-LCD device 51 includes a liquid crystal panel and a backlight device 70. The liquid crystal display panel includes lower and upper substrates 50 and 60 and an interposed liquid crystal layer 80. The upper and lower substrates 60 and 50 are called a color filter substrate and an array substrate, respectively.
The upper substrate 60 has color filters 61. The lower substrate 50 serves as the array substrate and includes TFTs (not shown), and transmissive and reflective electrodes 54 and 52 serve as a pixel electrode. The reflective electrode 52 surrounds the transmissive electrode 54 or has a light transmitting hole 53 having a dimension “ΔL”. The reflective electrode 52 includes a conductive material such as chrome (Cr), aluminum (Al) or tantalum (Ta), which has a high optical reflectivity, and therefor reflects the ambient light 74. The transmissive electrode 54, formed in the light transmitting hole 53, transmits the light 72 emitted from the backlight device 70.
The transflective LCD device 51 is operated as follows. First, in the reflective mode, the incident light 74 from the outside is reflected from the reflective electrode 52 and is directed toward the upper substrate 60. At this time, when the electrical signals are applied to the reflective electrode 52 by the switching elements (not shown), the arrangement of the liquid crystal layer 80 varies, and thus, the reflected light of the incident light 74 is colored by the color filter 61 and is displayed as colored light. Second, in the transmissive mode, light 72 emitted from the backlight device 70 passes through the transmissive electrode 54 (or transmitting hole 53). At this time, when the electrical signals are applied to the transmissive electrode 54 by the switching elements (not shown), arrangement of the liquid crystal layer 80 varies. Thus, the light 72 passing through the liquid crystal layer 80 is colored by the color filter 61 and displayed in the form of images with other colored lights.
FIG. 3 is a cross-sectional view of the conventional transflective LCD device. In FIG. 3, the color filter is not depicted because it does not affect the polarization state of the light. As shown, the conventional transflective LCD device 110 includes a first substrate 106 (an array substrate) and a second substrate 204 (a color filter substrate). A liquid crystal layer 300 that affects the polarization state of the light according to the applied voltages is interposed between the first substrate 106 and the second substrate 204.
On the surface of the first substrate 106 that faces the second substrate 204, a TFT (not shown), a transparent conductive electrode 150 (i.e., a pixel electrode) and a reflective electrode 108 (i.e., a pixel electrode) are disposed. Lower polarizer 102 is disposed on the other surface of the first substrate 106. Moreover, a lower retardation film (quarter wave plate; QWP) 104 having a phase difference λ/4 is positioned between the first substrate 106 and the lower polarizer 102. A backlight device 101 is adjacent to the lower polarizer 102. The lower polarizer 102, the lower retardation film 104, the first substrate 106, the transparent conductive electrode 150, and the reflective electrode 108 are all together referred to as a lower substrate 100.
On one surface of the second substrate 204 is a second retardation film, i.e., Quarter Wave Plate (λ/4 plate); referred to hereinafter as a second QWP 206. On the second QWP 206 is an upper linear polarizer 208. A transparent conductive common electrode 202 is on the other surface of the second substrate 204 facing the lower substrate 100. The common electrode 202, the second substrate 204, the second QWP 206, and the upper polarizer 208 are all together referred to as an upper substrate 200.
The second QWP 206 changes the state of the light. Namely, the second QWP 206 converts the linearly polarized light into the right- or left-handed circularly polarized light, and it also converts the right- or left-handed circularly polarized light into the linearly polarized light of which polarization direction is 45° or 135°.
The polarization state of the light of the conventional transflective LCD device described above will be explained hereinafter in accordance with each layer. FIGS. 4A and 4B illustrate the state of the light from the backlight device 101 through selected components of the conventional transflective LCD device 110 of FIG. 3 when in the transmissive mode. The conventional transflective LCD device has a normally white (NW) mode, i.e., the transflective LCD device displays a white color when a signal voltage is not applied.
FIG. 4A shows the state of the light from the backlight device in the transmissive mode when a signal voltage is not applied, i.e., when the TFT is turned OFF. The light from the backlight device enters the lower polarizer 102. In this case, transmissive axis of the lower polarizer is arranged perpendicular to that of the upper polarizer 208. Only the portion of the light that is parallel with the transmissive axis of the lower polarizer 102 passes through the lower polarizer 102 as linearly polarized light of which polarization direction is 45°. The resultant linearly polarized light is converted into left-handed circularly polarized light as it passes through the first QWP 104. Then, the left-handed circularly polarized light passes through the first substrate 106 and through the transparent conductive electrode 150 without any phase shift. Next, the left-handed circularly polarized light is converted into linearly polarized light of which polarization direction is 45° as it passes through the liquid crystal layer 300, this being due to a optical retardation λ/4 of the liquid crystal layer 300. The linearly polarized light then passes through the transparent conductive common electrode 202 and through the second substrate 204. As the linearly polarized light passes through the second QWP 206, the linearly polarized light is converted into left-handed circularly polarized light. Only the portion of the left-handed circularly polarized light that is parallel with the transmissive axis of the upper polarizer 208 passes through the upper polarizer 208. That is, about 50% of the left-handed circularly polarized light can pass through the upper polarizer 208. As a result, the LCD device produces a dark gray color.
FIG. 4B shows the state of the light from the backlight device in the transmissive mode when a signal voltage is applied, i.e., the TFT is turned ON: The liquid crystal does not affect the incident light, and thus the incident light passes through the liquid crystal layer without any change of polarization state. As depicted in FIG. 4B, the light from the backlight device 101 enters the lower polarizer 102. Only the linearly polarized light of the light of which polarization direction is 45° can pass through the lower polarizer 102. The resultant linearly polarized light is converted into left-handed circularly polarized light as it passes through the first QWP 104. Then, the left-handed circularly polarized light passes through the first substrate 106, through the transparent conductive electrode 150, and through the liquid crystal layer 300 without any polarization change. The left-handed circularly polarized light also passes through the common electrode 202 and through the second substrate 204 without any change of polarization state. The left-handed circularly polarized light is then converted into linearly polarized light by the second QWP 206. The polarization direction of this linearly polarized light is 45°. Therefore, the linearly polarized light is polarized perpendicular to the transmissive axis of the upper polarizer 208 and does not pass through the upper linear polarizer 208. Thus, the LCD device produces a black color.
FIGS. 5A and 5B illustrate the polarization state of the ambient light through selected components of the conventional transflective LCD device 110 of FIG. 3 when in the reflective mode.
FIG. 5A shows the state of the ambient light in the reflective mode when a signal voltage is not applied, i.e., the TFT is turned OFF. The ambient light illuminates the upper linear polarizer 208. Only the portion of the ambient light that is parallel with the transmissive axis of the upper polarizer 208 passes through the upper polarizer 208 as linearly polarized light (135° from x-axis of reference frame). The linearly polarized light is changed into right-handed circularly polarized light by the second QWP 206 which is parallel with x-axis of the reference frame. The left-handed circularly polarized light passes through the second substrate 204 and through the common electrode 202 without any polarization change. The right-handed circularly polarized light then passes through the liquid crystal layer 300 that has optical retardation (defined by (d·Δn) hereinafter) λ/4 which is parallel with y-axis of reference frame. The right-handed circularly polarized light is then converted into linearly polarized light of which polarization direction is 135° as it passes through the liquid crystal layer 300. The linearly polarized light is then reflected by the reflective electrode 108. The reflected linearly polarized light is converted back into a right-handed circularly polarized light as it passes through the liquid crystal layer 300. The right-handed circularly polarized light is then converted into a linearly polarized light of which polarization direction is 135° as it passes through the second QWP 206. The linearly polarized light is parallel to the transmissive axis of the upper polarizer 208, and thus passes through the upper linear polarizer 208. Thus, the LCD device produces light having a white color.
FIG. 5B shows the state of the ambient light in the reflective mode when a signal voltage is applied, i.e., the TFT is turned ON. In the ON-state, the liquid crystal layer 300 does not affect polarization state of the incident light. Thus, incident light passes through the liquid crystal layer without any change of polarization state.
Accordingly, the ambient light that passes through the upper polarizer 208 as linearly polarized light is converted into right-handed circularly polarized light by the second QWP 206. The right-handed circularly polarized light passes through the second substrate 204, through the common electrode 202, and through the liquid crystal layer 300. The right-handed circularly polarized light is then reflected by the reflective electrode 108, which causes the right-handed circularly polarized light to become converted into left-handed circularly polarized light with a phase shift of 180° via a mirror effect. The left-handed circularly polarized light then passes through the liquid crystal layer 300, through the common electrode 202, and through the second substrate 204. The left-handed circularly polarized light is then converted into linearly polarized light of having a polarization direction of 45° as it passes through the second QWP 206. The linearly polarized light is perpendicular to the transmissive axis of the upper polarizer 208, and as such does not pass through the upper linear polarizer 208. Thus, the LCD device results in a black color.
As described above, the conventional transflective TFT-LCD device has both the reflective mode and the transmissive mode such that it can be used in anywhere and anytime of the day. However, referring to FIG. 4A, the LCD device produces the dark gray color, unlike the FIG. 5A, although it should display a white color when the signal voltage is not applied. This is because about 50% of right-handed circularly light having passed through the second QWP 206 only can pass through the upper polarizer 208.
Therefore, since the difference of the brightness occurs between in the reflective mode and in the transmissive mode when the TFT is turned OFF, the definition and picture quality of the transflective LCD device are lowered. Accordingly, the transflective LCD device is designed more focusing on the reflective mode and cell gaps “d1” (see FIG. 3) of the reflective portion and “d2” (see FIG. 3) of the transmitting portion are substantially equal. Namely, the ambient light in the reflective mode passes through the liquid crystal layer twice due to reflection of the reflective electrode, while the light from the backlight device in the transmissive mode passes through the liquid crystal layer just once. Thus, the transflective LCD device cannot produce the pure white color when the signal voltage is not applied.
Moreover, the design of the conventional transflective LCD device focuses on the way that the length of cell gap and the optic axes of other components depend on the central wavelength band of the visible light (the green wavelength band, i.e., 550 nm). Therefore, the conventional LCD device does not properly control the ON/OFF-switch of the blue wavelength band or the red wavelength band. Furthermore, both lower and upper substrates are required in the conventional LCD device.
In other words, the transmissive axis of the lower polarizer is perpendicular to that of the upper polarizer in the conventional LCD device. Moreover, if the cell gap of the LCD device is designed focusing on the green wavelength band (centered at 550 nm), the LCD device precisely transmits or shut off the green wavelength band. However, the LCD device does not transmits or shut off the other bands substantially. These limitations cause a decrease in the switching ability of the LCD device.